Proper sidewall deflection is an important safety measure… all tires are designed with a static loaded radius (which is different than the tire radius)… that is the height from the(level) road surface to the center of the axle with its travel load on it… it maximizes the tires footprint on the road…
Weigh the truck and check the tires loading charts is the best and easiest starting point..
No scale, use the SLR measurement until you get a chance to scale it, If that measurement is to high you have too much air for the load… measurement to low you need to add more air to the tire… it puts you in the right ballpark to start with…